A Class C Hacked Keycard is used to unlock doors that require it. Upon unlocking a door, one keycard is consumed and the door is permanently unlocked (it can still be closed).
Class C is the most common of the three hacked keycard classes, in terms of both availability and usage. They are usually found in desks and occasionally in bathroom shelves, but the spawns are random.
Randomly generated rooms can contain doors that require a class C card. However, these rooms are always dead-ends that usually contain some useful items, so a hacked keycard is never required in a randomly generated floor.